1) Field of the Invention
The present invention relates to a method for switching a transmission route in an optical network in which nodes are connected through a plurality of transmission lines realizing a working route and a protection route, when a failure or the like is detected. The present invention also relates to an optical transmission device in which a transmission route is switched by using the above method.
2) Description of the Related Art
With the recent increase in the capacities of the networks, use of the WDM (Wavelength Division Multiplex) technology has been spreading to construction of metro area networks as well as the networks in backbone systems. In the current optical networks, the transmission distance and the capacity are increased by use of optical amplifiers. However, since optical amplification by use of an optical amplifier is repeated, the noise levels in optical signals become higher. Therefore, in order to further increase the transmission distance, transmission devices having a function of regenerative repeating are used.
The transmission devices having a function of regenerative repeating are repeaters which convert optical signals into electric signals, and thereafter convert electric signals into optical signals. The main factors of limiting the increase in the transmission distance are accumulation of noise (represented by the SNR) caused by use of the optical amplifiers, dispersion in optical devices, and deformation of optical signals caused by nonlinear effects. The SNR is the ratio of the signal component to the noise component. Therefore, there is a limit to transmission of optical signals by use of only optical amplifiers without conversion into electric signals. That is, it is necessary to convert the optical signals into electric signals, regenerate the original information from the electric signals, and produce noiseless optical signals for transmission. The transmission devices which regenerate and transmit optical signals as above are called regenerator stations.
In addition, in the optical networks, a plurality of transmission routes are arranged between nodes so that even when one of the plurality of transmission routes fails, optical transmission can be performed through another transmission route. For example, in the optical ring networks, nodes are circularly arranged, and transmission lines connecting each pair of adjacent nodes are doubled. In the case where counterclockwise transmission routes for optical signals at arbitrary wavelengths are used as working routes, clockwise transmission routes are used as protection routes.
Further, in the optical networks, the function of an optical ADM (add-drop multiplexer), which can add (insert), drop (extract), and let through (relay) optical signals at each wavelength, is used. When the function of the optical ADM is used in an optical network, it is possible to construct flexible networks by performing control operations, such as the add (insert), drop (extract), and through (relay) operations, on optical signals at each wavelength at each station arranged on the optical network. When a node having the function of the optical ADM is a receiver station, and detects occurrence of a failure at each wavelength or in each transmission line as a working route, the node acquires optical signals through a protection route. Such a function of switching the transmission route is called the optical protection function.
In optical protection, a fixed threshold of the optical level is used as a switch trigger. When the optical level falls below the threshold, the receiver station having the optical protection function determines that a line failure occurs, and switches the setting in the optical layer from the working route to a protection route. In this case, generally, recovery in 50 ms (milliseconds) is required. An example of an optical network having the above optical protection function is the OUPSR (Optical Unidirectional Path-Switched Ring), in which each transmitter station transmits main signals through both of a working route and a protection route. When a failure occurs in the working route, a receiver station determines loss of light on the basis of LOL (loss of light) detection by a photodetector (photodiode) provided in the receiver station. When the loss of light is determined, the route for use in transmission is switched to the protection route by using an optical switch or the like.
Incidentally, even in the optical networks in which a working system and a protection system are provided in each span between nodes, an instantaneous interruption of an optical signal occurs when the switching between the working system and the protection system is done. Therefore, reduction of the interruption time by improvement of the rising time of the optical amplifier has been proposed, for example, disclosed in Japanese Unexamined Patent Publication No. 2003-69455, paragraphs Nos. 0027 to 0028 and FIG. 3.
However, in the optical networks in which optical amplifiers are connected in multiple stages, optical noise accumulates. In addition, when an optical signal interrupts, the optical noise which is produced and accumulated by the optical amplifiers remains, and the input power at repeaters and a receiver station increases. In such cases, if only a fixed threshold of the optical level is used as the switch trigger for optical protection, it is possible to expect that loss of optical input (RIN) in a receiver station or the level alarm (LOL) of optical input into an optical detector cannot be detected. That is, it is difficult for the receiver station to detect a failure with reliability.